The present invention relates to a method and apparatus for registration control during the processing of workpieces. The invention is particularly useful in registration control during the production of images or other process performed with respect to printed circuit boards (PCBs) or other substrates, and is therefore described below with respect to such an application.
Registration control is important in many processing operations to be performed on workpieces. It is particularly important in processing operations requiring the precise matching of a film or an electronic image (i.e., the eletronic representation of an image) to a given substrate. Examples of the latter processing operations include contact printing of patterns in multi-layer PCBs, manufacturing of integrated circuits in microelectronics, and plotting of pre-press tooling panels. For example, manufacturing multi-layer PCBs involves the fabrication, and then the stacking, of up to 20 or 30 layers, in which each layer (commonly referred to as an inner layer) has its own previously generated conductor or other (e.g., through holes) pattern.
Manufacturing such multi-layer PCBs encounters a number of registration problems, particularly the following: (1) registration of the image of each layer with respect to those in the other layers; (2) registration of an image on one side of a layer with respect to an image on the other side of the layer; (3) registration of the image plotted on each side of the layer with respect to other tasks involved in processing the layer or the board, such as outer-layer processes, drilling holes, etc.; and (4) registration of the image on each side of the layer with respect to existing pre-drilled holes through the layer. These registration problems become increasingly more difficult to overcome as the component-density of the boards, wafers, or other workpieces increases, the number of layers increases, and/or the size of the workpiece decreases.
Various techniques for overcoming these types of registration problems are described in U.S. Pat. Nos. 4,829,375, 5,136,948, 5,164,742, 5,274,394, 5,403,684, 5,453,777, 5,459,941, 5,500,801, 5,548,372, and 5,699,742.
Most of the known techniques generally involve reorienting the substrate with respect to the machine, and then plotting the image from the image file. However, such known techniques have a number of drawbacks, particularly when plotting on PCBs or other substrates under conditions producing variations in thickness in the layer, or variations in thickness or in length among a series of layers. For instance, when a substrate having significant thickness is mounted on a cylindrical drum which is rotated with respect to the printing or plotting elements, the outer surface of the substrate is under tension thereby increasing its length, whereas the inner surface of the substrate is under compression thereby decreasing its length. An image plotted from an image file on the tensioned outer surface of the loaded layer will shrink when the layer is unloaded; whereas an image plotted on the compressed inner surface will expand when the layer is unloaded. These effects produce scaling changes between the plotted image relative to the image file. These scaling changes, which depend on the thickness of the layer and the loading conditions, introduce registration problems when printing images from individual image files on a plurality of overlying layers or on the opposite faces of the layers.
Another registration problem is encountered when imaging two sides of a layer, one after the other in the plotting machines which can not plot the two sides of the layer simultaneously. This problem is present with respect to both rotary-drum and flat-bed imaging or plotting machines.
A still further registration problem is introduced by dimensional changes of the substrate during processing. For example, temperature variations during processing induce thermal expansion or contraction of the substrate. Thus, thermal changes during the processing of different layers produce dimensional changes which generate registration errors when the layers are stacked to produce the multiple-layer PCB. Dimensional changes are also introduced by mechanical deformation of the layers during processing (e.g., mechanical pre-cleaning), deformation due to stress release (e.g., due to heating in the resist-coating step or due to laminate deformation after copper etching), etc. These registration problems become much more difficult to overcome when the dimensional changes are local and therefore not correctable by a global compensation.
An object of the present invention is to provide a method and apparatus for controlling a processing operation to be performed on a workpiece, particularly an imaging or printing operation to be performed on a PCB inner layer or other substrate, which method and apparatus particularly address some or all of the foregoing registration problems.
According to one aspect of the present invention, there is provided a method of controlling a processing machine to perform a processing operation on a workpiece, comprising: (a) determining the nominal locations of at least two sensible reference marks on the workpiece in terms of the coordinates of the workpiece; (b) loading the workpiece on the processing machine; (c) sensing, and measuring the actual locations of, the reference marks on the workpiece, when so mounted, in terms of the coordinates of the processing machine; (d) determining at least one geometrical transformation needed to transform the workpiece coordinates of the nominal locations of the reference marks to the processing machine coordinates of the actual locations of the reference marks; and (e) controlling the processing machine in accordance with the determined geometrical transformation.
It will thus be seen that the method as set forth above does not re-orient the workpiece with respect to the processing machine, as in some of the prior art techniques describe in some of the above-cited patents, but rather changes the workpiece coordinates to the processing machine coordinates after the workpiece has been mounted on the processing machine, and thereby compensates for changes in the workpiece, such as changes in the length or in the mounting position of the workpiece, when mounted on the processing machine.
According to further features in the preferred embodiments of the invention described below, the actual location or each of the workpiece reference marks is determined in step (c) by; fixing a sensing device to the processing machine for sensing the workpiece reference marks, with a reference point of the sensing device being at a known location with respect to a reference point of the machine; and vectorially adding the line vector from the machine reference point to the sensing device reference point, and the line vector from the sensing device reference point to the respective workpiece reference point.
More particularly, in the described preferred embodiment, the actual location of each of the workpiece reference marks is measured in step (c) by: (i) fixing a camera to the processing machine such that a reference point of the camera field of view is at a known location with respect to a reference point of the machine; (ii) actuating the camera to view a portion of the workpiece which includes the respective workpiece reference mark; (iii) measuring the location of a reference point on the respective workpiece reference mark in the camera field of view, relative to the camera field reference point; (iv) and vectorially adding the line vector from the machine reference point to the camera field reference point, and the line vector from the camera field reference point to the respective workpiece reference point to thereby determine the location of the respective workpiece reference point relative to the machine reference point.
According to another aspect of the invention, there is provided a method of controlling the plotting head of an imaging machine for producing an image on a substrate according to an image file, comprising: (a) determining the nominal locations of at least two sensible reference marks on the substrate in terms of the coordinates of the substrate; (b) loading the substrate on the imaging machine having a plotting head; (c) sensing, and measuring the actual locations of, the reference marks on the mounted substrate in terms of the coordinates of the imaging machine; (d) determining at least one geometrical transformation needed to transform the substrate coordinates of the nominal locations of the reference marks to the imaging machine coordinates of the sensed locations of the reference marks; (e) and controlling the plotting head to produce an image on the substrate in accordance with the determined geometrical transformation.
According to yet another aspect of the present invention, there is provided a method of processing a substrate comprising: (a) performing a first processing operation on the substrate; (b) mounting the substrate on a processing machine to perform a second processing operation thereon; (c) sensing the actual locations of preselected features on the substrate produced by the first processing operation; (d) and controlling the second processing operation in accordance with the sensed features.
According to further features of this aspect of the invention in the preferred embodiment described below, steps (c) and (d) are performed by: (i) determining, before step (b), the nominal locations of at least one reference feature on the substrate in terms of the coordinate of the substrate; (ii) after step (b), sensing, and measuring the location of, the reference feature in terms of the coordinates of the machine to perform the second operation on the substrate; (iii) determining at least one geometrical transformation needed to transform the substrate coordinates of the nominal location of the reference feature to the machine coordinates of the sensed location of the reference feature; and (iv) controlling the second processing operation in accordance with the determined geometrical transformation.
As will be described more particularly below, the second processing operation is thus controlled by the actual location of features on the substrate produced during the first processing operation, and thereby corrects any errors produced during the first operation performed on the substrate.
For example, the first operation may be drilling holes in accordance with a drilling file in a PCB inner layer; and the second processing operation may be printing a conductor image from a conductor pattern file, in which conductor pads are to coincide with drilled holes to enable the proper connections to be made to the conductor patterns in the various inner layers, e.g., by passing conductor pins through the holes. Thus, the printing operation is controlled by adjustment of the printing elements according to the actual location of some or all of the drilled holes in a way to bring the conductor pads to their proper positions relative to the drilled holes, thus eliminating misregistration between holes and pads. The same technique may be applied to correct misregistration between holes and pads where the pads are produced during the first processing operation, and the holes are to be produced during the second processing operation based on the sensed actual locations of the pads.
According to a still further aspect of the present invention, there is provided a method of controlling an imaging operation performed by a plotting head on the surface of a substrate having thickness, comprising: continuously measuring the thickness of the substrate; and continuously controlling the plotting head and the electronic image to compensate for variations in the thickness of the substrate.
According to a still further aspect of the invention, there is provided apparatus for controlling a processing operation on a workpiece carrying sensible reference marks at nominal locations on the workpiece in terms of the coordinates of the workpiece, comprising: a mounting device for mounting the workpiece to the apparatus; a processing head movable with respect to a workpiece when mounted on the mounting device; a sensing device carried by the processing head so as to move therewith with respect to a mounted workpiece, the sensing device being capable of sensing the workpiece reference marks and having a reference point which is at a known location with respect to a reference point on the processing head; and a data processor system processing the output of the sensing device for: (a) determining the locations of the reference marks on the workpiece in terms of the coordinates of the processing head; (b) determining the geometrical transformations needed to transform the workpiece coordinates of the nominal locations of the reference marks to the processing head coordinates of the sensed locations of the reference marks; and (c) controlling the processing operation in accordance with at least one of the geometrical transformations.
Further features and advantages of the invention will be apparent from the description below.